This invention relates to polyamides, and more particularly to improvement in thermally stabilized polyamide compositions. Still more particularly, the invention is directed to partially aromatic polyamides having improved thermal stability suitable for use in combination with thermal oxidative stabilizers, particularly copper-containing stabilizers. The invention may be further described as a method for improving the thermal stability of partially aromatic polyamide injection molding resins containing copper(I) stabilizer by reducing the level of carboxylic acid endgroups in the polyamide.
Polyamides generally exhibit a balance of thermal, strength and stiffness properties which make them suitable for many applications. The resins are particularly attractive for use in applications where resistance to chemical and thermal attack is required. Aliphatic polyamides, termed nylons, generally are readily processed thermally and have gained wide acceptance in the molding arts and in the extrusion arts, including fiber spinning and film extrusion. Many such polyamides find use in the form of yarn as tire cord and other applications where high tenacity and low shrinkage are needed.
Partially aromatic polyamides and copolyamides have been developed for use in high temperature applications, and crystalline and semi-crystalline copolyamides comprising at least about 40 mole % partially aliphatic terephthalamide units are known for their particularly good thermal properties and performance in demanding environments. However, such polyamides have relatively high melting points e.g., about 290.degree. C. or higher, and the degradation temperatures for some do not greatly exceed their melting points; accordingly, requirements for melt processing these polyamides are more rigorous and complex than those for polyamides such as nylon 6,6, melting at about 260-265.degree. C.
Fabricating high temperature, partially aromatic polyamides for the production of molded articles, extruded profile goods, laminates or the like, particularly when filled, requires processing the resin at temperatures very near the resin decomposition temperature, together with severe shear stress during molding or extrusion operations. Melt spinning operations such as are disclosed and described in U.S. Pat. No. 5,106,946 for producing fiber and yarn subject the resin to severe stress through application of high shear at high temperatures. Drawing operations at elevated temperatures, often used with fiber and film to develop crystallinity, may expose the resin to dry heat for extended periods. Good thermal stability is thus critically important to attaining good properties as well as to maintaining those properties in a variety of uses, particularly in demanding environments.
The art of stabilizing resins against deterioration through exposure to thermal oxidative environments is well-developed. The decomposition of aliphatic polyamides has been the subject of a great many studies, and numerous additives have been proposed for improving their thermal oxidative resistance, both during processing and while in use. Stabilizers act to inhibit the oxidation processes, preserving the aliphatic polyamide chain intact. The short-term thermal stability needed for most processing may be realized by incorporating a hindered phenolic antioxidant such as di-tertiary butyl cresol or any of the closely-related compounds and derivatives commonly used in the resin arts for these purposes. Stabilizer compositions comprising copper(I) halide and an alkali metal halide are also described in the art for use with polyamides, and the use of complex compounds comprising copper salts and diamines has also been disclosed for use with polyamide filaments. See U.S. Pat. No. 3,639,335. Dispersions of solid cuprous phthalate and potassium iodide have been used at levels corresponding to ca. 60 ppm copper to stabilize filaments comprising nylon 6,6 and copolymers thereof comprising minor amounts of hexamethylene isophthalamide, as shown in U.S. Pat. No. 3,457,325. Heat stabilizers comprising combinations of copper halides, alkali metal halides and phosphorus compounds have been employed for use in polyamide molding resins and the like, as shown for example in U.S. Pat. No. 4,937,276.
The aliphatic segments of high temperature, partially aromatic polyamides are subject to the same thermal-oxidative decomposition processes, and the thermal stabilizers for aliphatic polyamides have also been found useful with these polyamides. However, because partially aromatic polyamides generally require higher processing temperatures and otherwise are likely to be subjected to more severe conditions, the compounder may often find it necessary to use higher levels of stabilizers to adequately stabilize these high temperature polyamides.
Other modes of thermal oxidative attack may also occur in high temperature, partially aromatic polyamides. The aromatic acid moiety may undergo thermal decarboxylation, particularly at elevated temperatures, producing bubbles and voids in the molding. Stabilizers commonly employed with aliphatic polyamides may partially decompose thermally during processing at these elevated temperatures and form gaseous products that detrimentally affect the properties or appearance of molded and extruded goods. Where substantial degradation occurs, these byproducts may also result in splay formation in molded articles.
Copper compounds are known to be particularly facile aromatic acid decarboxylating agents, and adding such stabilizers to inhibit oxidation of the aliphatic portion of the polyamide may in fact promote thermal decomposition of the aromatic portion of the polymer. Melt extrusion, injection molding and melt spinning formulations that contain these stabilizers can cause polymer degradation with concomitant discoloration and substantial bubble formation. Minor amounts of additional components have been found to improve the thermal stability of copper-stabilized polyamides. See U.S. Pat. No. 5,447,980.
The art has continued to seek more effective stabilizing formulations for high temperature, partially aromatic polyamides. Though greater levels of stabilizers in combination with additional inhibiting compounds may be found to adequately stabilize aliphatic polyamide resins, these approaches likely will significantly increase costs, thereby tending to limit commercial acceptability. Moreover, the presence of additives, particularly in substantial quantities, often detrimentally affects the balance of mechanical properties of articles made from such formulations.
The art has not recognized the possibility of improving thermal stability by controlling the balance of endgroups or by using endcapping reactions to reduce the number of acid and amine endgroups when use of a copper-based thermal oxidative stabilizer is contemplated. For example, terephthalic acid:isophthalic acid:hexamethylenediamine copolymers are disclosed in U.S. Pat. No. 4,818,793, but there is no mention of the relative number of endgroups or of the use of endcap to reduce the number of acid and amine endgroups. In U.S. Pt. Nos. 5,081,222; 5,252,661; and 5,504,146 there are disclosed copolymers comprising terephthalic acid:hexamethylenediamine units in combination with caprolactam and/or adipic acid:hexamethylenediamine units, but there is no mention of the use of endcap to reduce the total number of acid and amine endgroups, and the polymers exemplified are said to contain approximately equivalent numbers of carboxyl and amino endgroups. In U.S. Pat. No. 5,109,106 the preparation of copolyamides from terephthalic acid and isophthalic acid or mixtures thereof with 2-methylpentamethylenediamine and optionally 2-ethyl-1,4-tetramethylenediamine is disclosed. These copolymers were characterized as having relative numbers of carboxyl and amino endgroups that were within 80 .mu.eq/g of each other and, by the fact that chain limiting endgroups of the cyclic amine type were generally below 40 .mu.eq/g, again showing no contemplation of controlling the level of carboxyl endgroups.
The preparation of copolyamides from terephthalic acid with mixtures of hexamethylenediamine, 2-methylpentamethylenediamine and optionally 2-ethyl-tetramethylenediamine is disclosed in U.S. Pat. No. 5,322,923. Although the patent describes analytical methods for determining endgroup content of the resins, stoichiometric control is limited to adjusting for amines losses encountered during the process. There is no mention of specific endgroup control, other than to limit the formation of cyclic amine endgroups. The preparation of copolyamides from terephthalic acid, optionally in combination with isophthalic acid, and mixtures of hexamethylenediamine and 2-methylpentamethylenediamine is disclosed in U.S. Pat. Nos. 5,378,800 and 5,500,473. Although numerous preparation examples are given, there is no specific mention of endgroup control.
The complexity of polyamide thermal oxidation processes is shown, in part, by the work of B. Lanska and J. Sebenda for lactam polymer samples where the oxidation mechanism was not diffusion limited. In Eur. Polym. J. Vol. 21, No. 10, pp. 891-894 (1985), the thermo-oxidation of lactam polymers at relatively low temperatures, 140.degree. C., was found to be related to the polymerization conditions and secondary reactions and structures occurring during polymerization at temperatures above 250.degree. C. In extending this lactam work (Eur. Polym. J. Vol. 22, No. 3, pp. 199-202 (1986)), along with an investigation of fibers from lactam polymers (J.M.S.-Pure Appl. Chem., A30 (9&10), pp. 660-678 (1993)), numerous conditions were said to influence the stability. Polymers prepared with acid catalysis were the least stable while base catalysis yielded the most stable polymers. Extraction of the catalysts actually reversed the relative stability of the two polymers. When equimolar amounts of endgroups were present, increasing the levels of both endgroups increased stability. When an excess of acid groups were present, exposure to oxidation conditions resulted in an increase in the number of acid groups and rapid degradation. When an excess of amine groups were present, exposure to oxidation conditions resulted in a decrease in the number of amine groups and slower degradation. Polymerization conditions in excess of 230.degree. C. led to a rapid decrease in stability, presumably due to products of side reactions.
Methods and compositions for providing adequately stabilized, high temperature polyamide resins that permit the compounder to use lesser amounts of stabilizing additives would provide a significant cost advantage over present stabilization methods. Further, with reduced levels of additives present, still further advantages may be seen including improvement in the overall balance of mechanical properties, thereby increasing the acceptability of such polyamides for a wider range of uses.